THREE-DIMENSIONAL LAGRANGIAN TURBULENT DIFFUSION OF DUST GRAINS IN A PROTOPLANETARY DISK: METHOD AND FIRST APPLICATIONS

In order to understand how the chemical and isotopic compositions of dust grains in a gaseous turbulent protoplanetary disk are altered during their journey in the disk, it is important to determine their individual trajectories. We study here the dust-diffusive transport using Lagrangian numerical simulations using the popular 'turbulent diffusion' formalism. However, it is naturally expressed in an Eulerian form, which does not allow the trajectories of individual particles to be studied. We present a simple stochastic and physically justified procedure for modeling turbulent diffusion in a Lagrangian form that overcomes these difficulties. We show that a net diffusive flux F of the dust appears and that it is proportional to the gas density (ρ) gradient and the dust diffusion coefficient Dd: (F = Dd /ρ x grad(ρ)). It induces an inward transport of dust in the disk's midplane, while favoring outward transport in the disk's upper layers. We present tests and applications comparing dust diffusion in the midplane and upper layers as well as sample trajectories of particles with different sizes. We also discuss potential applications for cosmochemistry and smoothed particle hydrodynamic codes.